Pertinent areas of the classification manual concerned with this type of invention are, among others, Class 208, Subclass 310 and Class 585, Subclasses 820, 701 and 738. In Gray Jr. et al, U.S. Pat. No. 4,476,345, an invention is disclosed in which a portion of one of the product streams in an isomerization process is used to wash a recycle gas stream to improve the quality of the isomerate products. The molecular sieve adsorbent of Gray is one which can be naturally occurring or synthetically produced comprising a three-dimensional crystalline-zeolitic aluminosilicate which will selectively, on the basis of molecular size of the pores, adsorb normal paraffins from the isomerized product from branched chained and/or cyclic paraffins. The molecular sieves have pore diameters of about 5 .ANG. and are exemplified by Zeolite A, specifically calcium 5A, which exhibits pore diameters ranging from about 3 to about 5 .ANG.. The teachings of this patent are herein incorporated by reference in regard to one of the molecular sieves of this invention which functions to selectively remove the normal paraffins from the other two paraffinic species, i.e., mono-methyl-branched paraffins and di-branched paraffins.
The Gray et al disclosure is an improvement upon an isomerization process as taught in Holcombe, U.S. Pat. No. 4,210,771. This is a process for the virtually complete isomerization of normal paraffin hydrocarbons in a feed stream consisting essentially of mixed normal and branched hydrocarbons, where the feed stream is passed first through an isomerization reactor and the products derived therefrom are passed to an adsorption section which separates normal from branched paraffins to form an isomerate having both di- and mono-branched paraffins. A recycle stream comprising nearly pure normal paraffins is usually recycled to exhaustion. Other disclosures which may be commensurate with Holcombe comprise U.K. Pat. No. 876,730 and U.S. Pat. No. 3,755,144 issued to Asselin.
The zeolitic molecular sieve employed in Gray et al and Holcombe may be selected from any adsorbent which selectively adsorbs normal paraffins based on the molecular pore size of the aluminosilicate. A particularly suitable zeolite of this type is a calcium exchanged A zeolite. Naturally occurring zeolite molecular sieves which could be substituted for calcium exchanged A zeolite include chabazite and erionite. The particular flow scheme of adsorption as taught by Holcombe U.S. Pat. No. 4,210,771 is herein incorporated by reference to show an operable multiple zeolite molecular sieve absorption means, to achieve proper adsorption-fill and desorption-purge. The Holcombe patent is completely silent as to arrangements of a multiple number of different sieves which may be present in the absorption separation technique. In fact, in the drawing of Holcombe, the adsorption bed systems, 44, 46, 48, and 50, are all comprised of a calcium 5A zeolite in the form of 1/16-inch cylindrical pellets. Branched paraffins, whether they be mono- or di-branched, flow through the adsorption bed while unbranched normal paraffins are adsorbed. After a purge of the adsorbed normal paraffins from the zeolite molecular sieve, the recycle stream is comprised nearly entirely of normal paraffins and recycle hydrogen. This is mixed with the incoming feed before charge to the isomerization zone.
A second Holcombe patent, U.S. Pat. No. 4,176,053, discusses a normal paraffin-isomerization separation process. By this technique, normal paraffins are isolated from a feedstock mixture comprising normal and branched paraffins at super atmospheric pressures using an adsorption system comprising at least four fixed adsorbent beds containing a calcium 5A molecular sieve. A stream is formed comprising vapor from void space purgings of the adsorbent and feedstock containing iso-paraffins and normal paraffins. The molecular sieve employed to separate normal paraffins from said stream is selected to adsorb only normal paraffins from a mixture of branched, cyclic and normal hydrocarbons in order to segregate the normal paraffins from the mixture and provide a normal paraffin recycle stream to insure isomerization of the normal paraffins to exhaustion.
In U.S. Pat. No. 3,836,455 issued to Blytas, the separation of methylpentane and 2,2-dimethylbutane (as contrasted with 2,3-dimethylbutane of the instant invention) is accomplished using an offretite zeolite. U.S. Pat. No. 4,251,499 issued to Nanne et al teaches that ferrierite sieves are effective for dividing substantially unbranched structures (n-paraffins) from mixtures of same (n-paraffins) with branched structures (both mono-methyl and di-branched paraffins). Such was the state of the art in 1981 although the instant invention has shown that this teaching is no longer accurate in regard to the adsorption capacity of ferrierite aluminosilicates.
These patents teach that it is most advantageous to recycle normal paraffins to thereby isomerize the same to the isomerate components comprising mono-methyl-branched paraffins and di-branched paraffins. These disclosures suggest that the isomerate will have a certain quantity of mono-methyl-branched paraffins derived from the isomerization zone. These mono-methyl-branched paraffins will indigenously have an inherently lower octane value than the di-branched paraffins.
In contrast, applicants have discovered a new and more efficient isomerization process utilizing a multiple number of molecular sieves whereby both normal paraffins and mono-methyl-branched paraffins are recycled to increase the relative quantity of di-branched paraffins in the isomerate. Using the specific multiple molecular sieve separation technique of this process, mono-methyl-branched paraffins are diminished in the isomerate and substantially increased in the recycle stream. In other words, this process increases the degree of branching existing within the isomerate by increasing the quantity of mono-methyl-branched paraffins in the recycle stream.
In summary, the select combinative molecular sieve separation zone of this invention comprises a first molecular sieve which will adsorb normal paraffins and a second molecular sieve which will adsorb normal paraffins and mono-methyl-branched paraffins. The separatory molecular sieves of the above cited patentees (except for U.S. Pat. No. 3,836,455 issued to Blytas) adsorb only normal paraffins while allowing mono-methyl-branched paraffins to remain and commingle with the isomerate product stream. Finally, neither of the two select molecular sieves of this invention adsorbs 2,3-dimethylbutane, which is passed to the isomerate product stream as a select octane blending component for gasoline. In Blytas, on the other hand, a process is disclosed which is based on offretite which excludes 2,2-dimethylbutane but has a channel size which is too large to distinguish between methylpentane and 2,3-dimethylbutane.